1. Field of the Invention
This invention relates to etchant solutions used in the manufacture of integrated circuits. More particularly, this invention relates to buffered HF etchant solutions which are used to etch silicon dioxide layers used in the manufacture of integrated circuits.
2. Description of the Prior Art
Buffered hydrofluoric acid (also known as BHF or buffered oxide etchant or BOE) is an aqueous solution containing HF and ammonium fluoride (NH.sub.4 F) and is commonly used today to etch silicon dioxide layers during the fabrication of integrated circuit component and like. Commercially available buffered oxide etchant solutions today generally contain from 4:1 to 100:1 by volume ratio NH.sub.4 F (40% by weight) solution to an aqueous HF (49% by weight) solution. These mixtures provide a high concentration of bifluoride (HF.sub.2.sup.-) ion species which is the active etching species of the BOE. These solutions also provide a high concentration of fluoride (F.sup.-) ion which allows the solubilization of silicon ion as SiF.sub.6 .sup.-2 and, thus, allows the etchant bath to be stable for a relatively long period of time.
However, a significant problem in the shipping and storage of these buffered oxide etchants is the fact that the high concentration of both ammonium (NH.sub.4 .sup.+) and bifluoride (HF.sub.2 .sup.-) causes ammonium difluoride (NH.sub.4 HF.sub.2) to crystallize out of these solutions at relatively high temperatures (e.g. 24.degree. C. for 4:1 by volume BOE). When this occurs, these solutions must be heated before use to solubilize the NH.sub.4 HF.sub.2 or the etchant solution will exhibit undesirable low etch rates. As a result of this phenomenon, these solutions must be shipped in heated trucks and stored in heated areas to prevent the crystallization from occurring.
It is possible to produce a solution which has a lower crystallization point and a similar etch rate to the standard BOE solutions by lowering both the concentration of HF and NH.sub.4 F in the solution (See U.S. Pat. No. 3,650,960 which issued to Strauss et al on Mar. 21, 1972). However, by lowering the HF and NH.sub.4 F concentrations the effective life of the etchant is shortened since the solubilization of the silicon dioxide involves the consumption of HF.sub.2.sup.- ions from the solution.
Thus, it is an objective of this invention to provide solutions which are similar in HF and F.sup.- concentration to existing BOE solutions, thus having an effective long life, yet without the undesirable relatively high crystallization points associated with such standard solutions.
It is also an objective of this invention to produce buffered hydrofluoric acid solutions which etch at rates comparable to existing BOE solutions without the disadvantages associated therewith.
Still another object of the present invention is to provide buffered etchant solutions containing polyammonium fluoride salts and hydrogen fluoride which may be stored for extended periods of time at room temperature.
A further object of the present invention is to provide an improved process for etching silicon dioxide layers used in manufacture of integrated circuit components and the like where the etching operation may occur at relatively high etching rates and at room temperature.